A known type of optical scanner uses a rotating holographic disk as a beam deflecting element. The disk includes a circular glass substrate which supports an annular thin film divided into individual beam deflecting generally sectorial areas or facets. The thin film is an exposed and developed photosensitive material, such as a silver halide emulsion or a dichromated gelatin. The original of each facet in the annulus is exposed using known off-axis, holographic techniques. According to these techniques, two beams of coherent light, a reference beam and an object beam, are directed at a film of unexposed, photosensitive material. The overlapping beams create optical interference patterns in the layer of photosensitive material. These interference patterns are fixed by developing the material using conventional techniques suitable for the particular photosensitive material employed. If the material is then illuminated with a reconstruction beam which is the conjugate of the original reference beam, the thin film will diffract or bend part of the reconstruction beam to produce the conjugate of the original object beam.
For purposes of this description, a conjugate of a light beam may be defined as follows. All the rays of a conjugate beam are opposite to the rays of the original beam. That is, if rays in the original beam diverge from a single point, then rays in the conjugate of this beam will travel in the opposite direction and converge to that same point.
If the thin film is moved relative to the reconstruction beam, the conjugate of the object beam will sweep through an arc. The particular path followed by the conjugate beam is a function of the relative orientation of the original reference beam and original object beam at the time of exposure of the initial facet. By using different angles and orientations of original reference beams and original object beams, reconstructed object beams following different paths can be generated by different facets. Arrays of beam-folding mirrors can be located in the paths of the reconstructed object beams to redirect the object beams into complex, omni-directional scan patterns.
Multi-faceted holographic disks of the type described above can be made by exposing individual, oversized sheets of photosensitive material and by developing these sheets separately. Individual facets of the desired size and shape can be cut from the sheets and bonded to the clear glass substrate with a suitable adhesive material.
The steps of individually exposing a separate sheet of photosensitive material for each facet, cutting the facet to the correct dimensions, positioning the facet in the correct place on the clear glass substrate and bonding the facet to that substrate are obviously time consuming, labor intensive and subject to errors. These factors make this "cut and paste" type of disk fabrication unsuitable for anything other than extremely limited quantities of disks. A disk made by the "cut and paste" method described above is normally used only as a master disk.
To provide large quantities of holographic disks, the master disk itself may be used in a multi-step copying process, sometimes referred to as a step and repeat process. In a step and repeat process, an annular thin film of photosensitive material (a target disk) is placed face to face with the thin film of the master disk. All facets, except one, on the master disk are masked from any ambient light. The unmasked facet is illuminated using a collimated reference beam directed at the surface of the master disk at the same angle as the reference beam originally used in producing the facet on the master disk. When this reference beam is transmitted through the exposed facet on the master disk, the exposed facet will separate the beam into a zero order component, which is basically an extension of the reference beam, and a first order component, which follows the path of the original object beam. The zero and first order components of the beam will interfere in the previously undeveloped thin film to form an interference pattern in the target film. The area of the target film in which the interference pattern is formed corresponds to the area of the unmasked facet on the master disk.
Once a facet has been generated in the target disk using these steps, the exposed area is masked from any light and a different facet on the master disk is unmasked. The interference pattern recorded in this next facet is copied into the target film using a reference beam having the same orientation as the reference beam originally used to generate this next facet.
These steps are repeated one facet at a time for each facet in the master disk until the interference pattern of each facet has been copied into the target film. At this point, the target film is developed in a single processing operation.
While this "step and repeat" method is superior to the cut and paste method for manufacturing holographic disks in large quantities, it still has disadvantages. The step and repeat method takes an undesirable amount of time since each facet must be exposed in a separate operation, the facet area masked must be changed for each exposure operation, and; the orientation of the reference beam may have to be changed for successive exposure operations. All of these operations require intervention by a human operator or, as an alternative, a highly automated system capable of performing such operations. The costs of developing an automated system diminish the attractiveness of that alternative.